tkinter display new frame from menu

I'm trying to write my fisrt script in python & tkinter.

I block to display a new frame from a function (def onDisplay) when a click is made from menu bar button, but nothing append. No error is display :-(

The new frame is made with dynamic checkboxes from text files: txt file:

item1
item2
...
item100

A screen of the GUI:

Here my code:

from tkinter import Tk, Frame, Menu, Checkbutton, Text, TOP, BOTH, X, N, LEFT, BooleanVar
from tkinter.ttk import Frame, Label, Entry
import glob

class Example(Frame):

    def __init__(self):
        super().__init__()

        self.initUI()
        #self.display_srv()

    def initUI(self):

        self.master.title("Submenu")

        menubar = Menu(self.master)
        self.master.config(menu=menubar)

        fileMenu = Menu(menubar)

        submenu = Menu(fileMenu)
        submenu.add_command(label="lst1", command=self.onDisplay)
        submenu.add_command(label="lst2")
        submenu.add_command(label="lst3")
        fileMenu.add_cascade(label='Listing', menu=submenu, underline=0)

        fileMenu.add_separator()

        fileMenu.add_command(label="Exit", underline=0, command=self.onExit)
        menubar.add_cascade(label="File", underline=0, menu=fileMenu)

    #The frame i tried to display
    def onDisplay(self):
        self.master.title("display it")
        self.pack(fill=BOTH, expand=True)
        frame1 = Frame(self)
        frame1.pack(fill=X)
        path = '/root/liste/*.txt'
        files=glob.glob(path)
        count = 0
        for file in files:
            with open(file, 'r') as lst_file:
                for item in lst_file:
                    # Need to split all item by 10
                    Checkbutton(self, text=item.rstrip()).grid(row=count//10, column=count%10)
                    count += 1


    def onClick(self):
        if self.var.get() == True:
            self.master.title("Checkbutton")
        else:
            self.master.title("")

    def onExit(self):

        self.quit()


def main():

    root = Tk()
    root.geometry("850x550+300+300")
    app = Example()
    root.mainloop()


if __name__ == '__main__':

Many thanks for any help

Regards,